When coupling RF power via an RF electrode (for example, in a plasma processing apparatus), a voltage standing wave, VRF(r), occurs on the RF electrode surface. The voltage standing wave is typically radial (usually with azimuthal symmetry) with its maximum at the electrode center (r=0). Its current conjugate, IRF(r), has its minimum value at r=0. A problem of energy spatial distribution arises when the flat-electrode RF wavelength (not its free-space wavelength) becomes shortened to a dimension within about ten times the electrode radius. However, this relatively flat-electrode RF wavelength may still produce poor energy spatial distribution when dimensioned greater than or less than 10×.
When the RF frequency becomes high, for example, in the VHF range (about 30 MHz to about 300 MHz, e.g., 100 MHz), and is used in conjunction with plasma processing of large format wafers (or substrates), the observance of non-uniform spatial energy distribution becomes readily apparent. For example, the voltage at r=center (of the substrate) becomes noticeably higher than the voltage at r=edge (of the substrate). Since the problem is electromagnetic, the non-uniformity has a current-conjugate as well. The r=edge RF current becomes noticeably higher than the r=0 RF current (RF current is 0 at r=0). The standing wave energy spatial distribution problem primarily results from the voltage standing wave effect, causing a stronger capacitive mode in the center; and edge skin effect, causing a stronger inductive mode at the edge.
This irregular spatial energy distribution results in non-uniform plasma distributions, and as a result, irregular substrate processing characteristics. Therefore, an improved apparatus and method for improving RF energy uniformity is needed.